Electronic devices typically include a variety of common circuit components, including passive components such as resistors, inductors, and capacitors, as well as active components such as transistors and integrated circuits. Passive components such as inductors or capacitors are designed to store energy, and resistors are designed to resist the flow of applied current to a specified degree. Although a wide variety of passive components exist, most inductors are still coils of copper wire that are configured to have a specified opposition to changes in current flow, and most capacitors comprise conductive plates separated by an insulator and are configured to provide a specified opposition to change in voltage across the plates.
But, passive components in the real world do not have only those desired electrical characteristics described above. Every inductor has some resistance and a small amount of capacitance, and every capacitor has a small effective inductance and resistance. Although it is difficult to measure the resistance or inductance present across a capacitor because it includes a nonconductive layer, various methods of measuring and calculating an equivalent series inductance and an equivalent series resistance of a capacitor have been developed and are useful in characterizing capacitors. Equivalent series inductance, or ESL, is also sometimes called parasitic inductance, indicating that although it is present it is not desired.
One common application for capacitors is as a voltage stabilizing component that helps to maintain an applied voltage in the event of a voltage disruption or fluctuation. In order to efficiently and quickly supply its charge to maintain a voltage level in case of fluctuations, a capacitor must have a low equivalent series inductance so that the current supplied by the capacitor can change rapidly. Also, a low equivalent series resistance is desirable in that it results in less power being dissipated in the capacitor and more efficient transfer of the stored energy to maintain an applied voltage.
These voltage stabilizing capacitors are often called bypass capacitors, and are typically mounted very near the components for which they are intended to provide voltage stability. The close physical proximity is in large part intended to reduce the loop area through which supplied current must flow, resulting in a lower inductance in the capacitor's voltage supply path. As the clock speeds of computerized systems continues to increase, it becomes more difficult for capacitors with high equivalent series inductance or ESL to provide current to maintain a voltage as rapidly as the various electronic devices can demand current, and so close attention is paid to controlling and minimizing the ESL of capacitors used in such applications. It is therefore desirable to have a capacitor that is designed to provide a low equivalent series inductance or ESL.